Bio-threat detectors are used to monitor the ambient air to detect the presence of potentially harmful pathogens. In general, air is drawn into an air collection and detection system where the particulates in the air are evaluated. Airflow into the system is typically generated by a fan or blower within the apparatus. One method of evaluating the airborne particles is to continuously monitor the air and the individual molecules within a given airflow. Some detection methods include the use of lasers to scan the air path in order to interrogate the particles passing through. A harmless particle, such as a dust particle, can be discriminated from a harmful particle, for example an anthrax spore, because each different type of particle reflects a different wavelength of light. The laser light reflected of the passing particles is matched to database of known harmful wavelengths. When a harmful wavelength is detected, the trigger signals that a potential pathogen is present.
Another method of evaluating the airborne particles within the airflow entering the system is to collect the airborne particles within a fluid sample and then analyze the fluid sample. One such device for collecting airborne particles in a fluid sample is a liquid impingement unit.
FIG. 1 illustrates a conventional liquid impingement unit. The liquid impingement unit 10 includes a collection vessel 12, an air nozzle 14, and a vacuum tube 16. The collection vessel 12 is hermetically sealed by a lid 18. The lid 18 is configured such that a first end of the vacuum tube 16 and a first end of the air nozzle 14 are sealed within the collection vessel 12. Air is forced out of the first end of the air nozzle 14 and into the collection vessel 12. The first end of the air nozzle 14 is positioned above a buffer solution 20 such that air output from the air nozzle 14 impinges the collection buffer solution 20. In this manner, airborne particles within the air are forced into the buffer solution 20. To prevent excess pressurization within the sealed collection vessel 12, the vacuum tube 16 removes air from within the collection vessel 12.
One disadvantage of conventional liquid impingement units is that the high air flow rate necessary to successfully implant the airborne particles within the buffer solution causes an increase in the evaporation rate of the buffer solution within the collection vessel. The evaporation rate is influenced by many factors, including, but not limited to, the surface area of the buffer solution-to-air interface within the collection vessel and the airflow rate out of the air nozzle. The vaporized liquid particles are removed from the collection vessel via the vacuum tube. A given fluid sample including the collected particles is generated over a given period of time, for example anywhere from about 10 minutes to 5 hours of continuous operation by the liquid impingement unit. The liquid impingement unit will lose a portion of the buffer solution due to evaporation and removal of the evaporated particles when the air is vacuumed from the collection vessel. For example, initially 10 ml of buffer solution is added to the liquid impingement unit, but after 5 hours of operation the volume of buffer solution remaining may be only 1 ml.
Over a period of time, the volume of the buffer solution within the collection vessel drops below a minimum operation level, at which point, the air collection process must be stopped either to removing the remaining buffer solution, including the collected particles within, or to add additional buffer solution to the collection vessel so that the total volume of buffer solution is above the minimum operation level. Either solution requires disruption of the air collection process, the manual removal of the hermetic lid from the collection vessel, and either the manual removal of the remaining buffer solution or the manual addition of more buffer solution to the collection vessel.
Due to evaporation of the buffer solution, operation of conventional liquid impingement units are limited in the amount of collection time that can be achieved for a given initial volume of buffer solution. Either the collection time is limited, which constrains the protocols of any system utilizing the liquid impingement unit, or additional buffer solution needs to be manually added into the collection vessel.